Department of Electrical Engineering and Computer Science Disk subsystem performance can be dramatically im-proved by dynamically ordering, or scheduling, pending re-quests. Via strongly validated simulation, we examine the impact of complex logic & to-physicaJ mappings and large prefetching caches on scheduling effectiveness. Using both synthetic workloads and traces captured from six differ-ent user environments, we arrive at three main conclusions: (I) Incorporating complex mapping information into the scheduler provides only a marginal (less than 2YO) decrease in response times for seek-reducing algorithms. (2) Algorithms which effectively utilize prefetching disk caches provide sig-nificant performance improvements for workloads with read sequentiality. The cyclical scan algorithm (C-LOOK), which always schedules requests inascending logical order, achieves the highest performance among seek-reducing algorithms for such workloads. (3) Algorithms that reduce overall position-ing delays produce the highest performance provided that they recognize and exploit a prefetching cache.
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